How big would such an aquifer need to be to be picked up by SHARAD? We know there is a fair bit of relatively low lattitude ice a few meters beneath the martian surface, so wouldn't we expect to have seen some evidence if aquifers if this were happening in the present day?

I'm curious about this too, and my ignorance of the workings of ground penetrating radar knows no bounds. Maybe someone can enlighten us. Can SHARAD and MARDI distinguish between ice and liquid water? (A seismic survey - now that would be another thing.)

A very quick bit of reading tells me that SHARAD has avertical resolution of 15 to 20 meters. The electrical ( and hence radar) properties of water are different from those of ice so at a very very ill informed guess I'd say that SHARAD probably could distinguish between the two. So assuming my guess is near the mark the question is what is the vertical extent of these possible aquifers?

We know that Mars has plenty of H2O, and that like any other world it gets warmer as you go down, so I think their must be liquid water at some depth on Mars (to be drilled for perhaps by future inhabitants). Of course when it has the temerity to appear at the surface it immediately freezes and pretty soon sublimes away, so we're looking at the dry rind of a wet planet.How near the surface the liqiud phase gets and how often are great questions. I hope we get answers from the current generation of Mars explorers.

We know that Mars has plenty of H2O, and that like any other world it gets warmer as you go down, so I think their must be liquid water at some depth on Mars (to be drilled for perhaps by future inhabitants). Of course when it has the temerity to appear at the surface it immediately freezes and pretty soon sublimes away, so we're looking at the dry rind of a wet planet.How near the surface the liqiud phase gets and how often are great questions.

If temp rises by ~30 C with every kilometer, then liquid water should be at about three kilometers down.

Differing values have been reported for the average temperature on Mars, with a common value being −55 °C. Surface temperatures have been estimated from the Viking Orbiter Infrared Thermal Mapper data; this gives extremes from a warmest of 27 °C to −143 °C at the winter polar caps. Actual temperature measurements from the Viking landers range from −17.2 °C to −107 °C.

If that's true, then water could very possibly be just meters below the surface in some areas during daytime hours based on the lander estimates and could reside as liquid on the surface (if it was to get there) for extended periods according to the oribiter.

Edit: I guess I overlooked your last line. Yes, in this sense, it would be rare for liquid water to ever exist on the surface of Mars. However, honestly I have no idea what temperature water would need to be to exist as liquid. I will have to calculate that out.

That paper is 10 years old, and many other studies since have suggested alternative mechanisms for those landslides - including dry dust avalanches, CO2 sublimation etc. The wet theory behind them is less popular than a decade ago.

Den and ZLD - you're both forgetting one vital part of this puzzle. Air Pressure.

Surface conditions on Mars - at the very warmest - are playing around at the triple point of water. There would only be a tiny temperature window between ice melting, and evaporating away.

I never forget that. But with having positive Celsius temps a few *kilometers* down means than any vapor has ample time to re-freeze while it creeps through pores towards surface. I think over time this creates an extensive subsurface layer of permafrost, nearly airtight and impenetrable for water vapor from below.

Edit: I guess I overlooked your last line. Yes, in this sense, it would be rare for liquid water to ever exist on the surface of Mars. However, honestly I have no idea what temperature water would need to be to exist as liquid. I will have to calculate that out.

Water needs 0 C or 273 K to become liquid. Even on Mars. Melting temp depends only weakly on pressure.

Water needs 0 C or 273 K to become liquid. Even on Mars. Melting temp depends only weakly on pressure.

That isn't really all that true. Pressure can play a large part in the melting point, especially when you consider other factors such as water purity. Seawater for instance, freezes at -2C which is quite a big difference. Also, as pressure increases, melting points definitely do decrease, which is what allows Lake Vostok to remain (likely) in liquid form. Similarly, aquifers could exist at somewhat shallow (>1km) depths under Martian soil so long as the pressure exists. What would need to be determined then is, at what depth does this pressure exist?

Secondly, I never brought up the gullies. I am fully aware of the current hypothesis surrounding them. I was at work when I posted that link and didn't have a chance to fully read it. I only posted it because it seemed to have some information that related to this topic and I apologize for it not suiting that goal entirely.

That may be true but the boiling point of water depend quite a bit on pressure. And under most ambient Martian pressure conditions the boiling point is a fraction of a degree above and sometimes equal-to your melting point. As a result you are never going to have liquid water beyond a few fleeting moments under extremely rare conditions.

Best to leave your goldfish at home there's no stable liquid water on Mars.

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